Subtropical Acidic Soils Research Articles

Prevention values for copper (low tier approach) in subtropical acidic soils.

Copper is a trace element in plants and animals whose importance can be understood due to its role in different essential metabolic processes. Anthropogenic activities such as agriculture and mining are potential sources of pollution due to the emission of copper into the environment. Brazilian legislation ties soil quality to guideline values, among which the Prevention Value indicates the critical environmental limit for trace elements. The aim of this study was to obtain PVs for copper for two subtropical soils (Cambisol and Nitisol), given that the pedological richness was not considered when deriving the PVs contained in the federal normative. Reproduction assays followed ISO guidelines with the earthworm species Eisenia andrei and Perionyx excavatus, the enchytraeids Enchytraeus crypticus and Enchytraeus bigeminusand the springtails Folsomia candida and Proisotoma minuta. Results showed that the sensitivity of the organisms was greater in Cambisol. The most sensitive species were the earthworms, especially Perionyx excavatus (EC50 = 67.83 in Cambisol; EC50 = 264.96 in Nitisol). The springtails, on the other hand, were the least sensitive to contamination. These findings reinforce the need to include organisms from different ecological groups in ecotoxicological assessments. It was also observed that the PV adopted in federal legislation (= 60mgkg-1) is in fact protective for the species and soils we evaluated, since the PVs we obtained based on the EC50 were 346.74mgkg-1 in Nitisol and 134.05mgkg-1 in Cambisol. It is important to note that our results do not exclude the need for evaluations with other subtropical soils, given the influence of their properties on the toxicity and bioavailability of copper to soil organisms.

Evaluation of Biochar and Inorganic Fertilizer on Soil Available Phosphorus and Bacterial Community Dynamics in Acidic Paddy Soils for Different Incubation Temperatures

The composition of microbial communities and the functioning of ecosystems are greatly influenced by the nutrient inputs. Despite this fact, our knowledge regarding the impact of phosphorus (P) inputs on soil P availability and microbial community structures in subtropical acidic soils remains limited. We hypothesized that diverse P inputs, incubation temperatures, and soil types could significantly alter soil P availability and microbial communities. To address this gap, we conducted a laboratory incubation experiment, investigating the effects of biochar and inorganic P amendments on soil available P, soil pH, acid phosphatase enzymes, and bacterial abundance. We employed two different incubation temperatures (15 °C and 25 °C) using acidic paddy soil and red soil from the subtropical Southern China region. Our results indicate a notable increase in soil pH, reaching 37% and 39% at 15 °C and 40% and 40.6% at 25 °C, respectively, following the application of biochar and inorganic P amendments in paddy soil. In the case of red soil, we observed pH increases of 41% and 43% at 15 °C and 44% and 45% at 25 °C after the application of biochar and inorganic P amendment, respectively. The addition of inorganic P amendment resulted in the highest available P contents in paddy soil, reaching 111.47 mg/kg at 15 °C and 100.17 mg/kg at 25 °C, respectively. However, Proteobacteria decreased after inorganic P addition, which showed that P might not be the only limiting nutrient for various bacterial communities. Bacterial diversity and richness indices were found to be higher after biochar application in both soils. Gemmatimonadetes, Acidobacteria, and Actinobacteria were found to be strongly influenced by incubation temperatures, whereas most of the top abundant bacterial phyla, such as Gemmatimonadetes, Actinobacteria, Chloroflexi, Acidobacteria, Planctomycetes, Firmicutes, Patescibacteria, and Bacteroidetes, were highly dependent on soil type. At the genus level, various important P solubilizing genera (Pseudomonas, Bradyrhizobium, Streptomyces jietaisiensis, Massilia) significantly increased after biochar and inorganic P addition in both soils. The correlation analysis proved that P-solubilizing genera were significantly associated with changes in soil pH, as well as soil available P after biochar and inorganic P addition. Conclusively, in a short-term incubation experiment, inorganic P amendment greatly increased the soil pH and available phosphorus contents compared to biochar and control treatments; however, the microbial community was observed to be strongly associated with biochar application, soil type, and incubation temperature.

Bacillus velezensis SQR9 inhibition to fungal denitrification responsible for decreased N2O emissions from acidic soils

Tropical and subtropical acidic soils are hotspots of global terrestrial nitrous oxide (N2O) emissions, with N2O produced primarily through denitrification. Plant growth-promoting microbes (PGPMs) may effectively mitigate soil N2O emissions from acidic soils, achieved through differential responses of bacterial and fungal denitrification to PGPMs. To test this hypothesis, we conducted a pot experiment and the associated laboratory trials to gain the underlying insights into the PGPM Bacillus velezensis strain SQR9 effects on N2O emissions from acidic soils. SQR9 inoculation significantly reduced soil N2O emissions by 22.6–33.5 %, dependent on inoculation dose, and increased the bacterial AOB, nirK and nosZ genes abundance, facilitating the reduction of N2O to N2 in denitrification. The relative contribution of fungi to the soil denitrification rate was 58.4–77.1 %, suggesting that the N2O emissions derived mainly from fungal denitrification. The SQR9 inoculation significantly inhibited the fungal denitrification and down-regulated fungal nirK gene transcript, dependent on the SQR9 sfp gene, which was necessary for secondary metabolite synthesis. Therefore, our study provides new evidence that decreased N2O emissions from acidic soils can be due to fungal denitrification inhibited by PGPM SQR9 inoculation.

Coupled Effects of Reduced Chemical Fertilization and Biochar Supplementation on Availability and Transformations of Nitrogen and Phosphorus in Vegetable Farmland Soil: An In Situ Study in Southern China

Reduced fertilization technology is an eco-friendly strategy to minimize nitrogen (N) and phosphorus (P) surpluses and losses in vegetable production. However, little is known about the performance of chemical fertilizer reduction when supplemented with palm silk biochar (PSB) in subtropical acid soils. A short-term (60 d) field investigation under conditions of in situ incubation was conducted in vegetable farmland in southern China. The treatments included no fertilization (Control), 100% conventional fertilization (CF100), 90% conventional fertilization plus 10% PSB-based fertilization (CF90B10), 85% conventional fertilization plus 15% PSB-based fertilization (CF85B15), and 80% conventional fertilization plus 20% PSB-based fertilization (CF80B20). The CF90B10, CF85B15, and CF80B20 treatments had the same inputs of total N and P as the CF100 treatment. Reduced chemical fertilization generally decreased the soil NH4+-N regardless of the PSB substitution rate (10%, 15%, or 20%), incubation condition (top-covered or top-open: preventing or allowing the leaching process, respectively), and sampling time (1 day or 60 days). Conversely, compared with the CF100 treatment, both the CF85B15 and CF80B20 treatments did not lead to a significant decrease in the NO3−-N concentration in soil under top-open incubation conditions, but significantly (p < 0.05) increased soil NO3−-N under top-covered incubation conditions. The CF80B20 treatment significantly (p < 0.05) decreased soil Olsen-P in comparison with the CF100 treatment, regardless of the incubation condition and sampling time. After applying chemical fertilizer in combination with PSB, soil net ammonification and N mineralization tended to be reduced considerably, with substantial reductions of 39–76% and 24–45%, respectively; reversely, soil net nitrification was stimulated by an increased PSB substitution rate. As the rate of chemical fertilization decreased, the trends in NH4+-N and NO3−-N losses from the soil were similar to the trends observed in soil net ammonification and net nitrification, respectively. Additionally, there were no significant differences in the soil net P mineralization and Olsen-P loss between chemical fertilization alone and in combination with PSB application. Generally, the partial substitution of chemical fertilizer with PSB at a low application rate may not substantially reduce plant-available NO3−-N and Olsen-P. It can also contribute to the sustainable availability of N and P in vegetable farmland soil via a variety of transformation processes, such as mineralization, immobilization, and loss.

Differential responses of soil nitrogen‐oxide emissions to organic substitution for synthetic fertilizer and biochar amendment in a subtropical tea plantation

AbstractTropical and subtropical acidic soils have been well documented as hotspots of global soil nitrogen (N) oxide (i.e., nitrous oxide (N2O) and nitric oxide (NO) emissions). While the effectiveness of possible mitigation options has been extensively examined in croplands, little is known about their effectiveness in reducing N‐oxide emissions from acidic soils of rapidly expanding tea plantations in China. Here, we conducted a 2‐year field experiment to investigate how organic substitution for synthetic fertilizer and biochar amendment affect soil N‐oxide emissions from a subtropical tea plantation. Across the 2‐year measurement period, full organic substitution for synthetic fertilizer significantly increased N2O emissions by an average of 17% while had a lower NO emission compared to synthetic fertilizer alone. Our global meta‐analysis further revealed that full or partial organic fertilizer substitution resulted in a 29% (95% confidence interval: 5%–60%) increase of N2O emissions from acidic soils. In contrast, irrespective of fertilizer type, biochar amendment significantly reduced N2O emissions by 14% in the first but not second experimental year, suggesting a transient effect. The trade‐off effect of full organic substitution on N2O and NO emissions may be attributed to the favorable conditions for N2O production due to the stimulated activity of nitrifiers and denitrifiers. The suppression of N2O emission following biochar amendment was probably due to promoted further reduction of N2O to dinitrogen. The fertilizer‐induced emission factor (EF) of N2O (2.1%) in the tea plantation was greater than the current IPCC default value, but the EF of NO (0.8%) was comparable to the global estimate. Taken together, while biochar amendment could have mitigation potential, cautions are needed when applying organic substitution for synthetic fertilizer as mitigation options for acidic soils as hotspots of N‐oxide emissions.

Continuous Cropping Affects Gross Nitrogen Transformations in Subtropical Acidic Soils Under Greenhouse Cultivation

Relationship between gross N transformation rate and continuous cropping years in the greenhouse soil is largely unknown. This study aimed to investigate the mechanism of the gross N transformation rate changes in greenhouse subtropical acidic soils under different continuous cropping years. The 15N isotope tracer method was used to study the effects of greenhouse soil under 4 years of continuous cropping (C4), 5 years of continuous cropping (C5), and 7 years of continuous cropping (C7) of lily (Lilium brownii), respectively, on soil N transformation with the N tracer model. The results showed that the main soil N transformation processes were N mineralization, autotrophic nitrification, and NO3−-N immobilization under the three continuous cropping years. After continuous cropping, the gross N mineralization rate, autotrophic nitrification rate, and NO3−-N immobilization rate increased with the increase of continuous cropping years. Specifically, the gross N mineralization rate was 0.62~0.85 mg N kg−1 day−1, the autotrophic nitrification rate was 1.08~3.24 mg N kg−1 day−1, and the NO3−-N immobilization rate was 0.30~0.62 mg N kg−1 day−1. Moreover, the positive difference between the autotrophic nitrification rate and immobilization rate of NO3−-N became greater with the increase of continuous cropping years, which might have been the main reason for the accumulation of NO3−-N in greenhouse subtropical soils. These findings contribute to the understanding of the mechanism of soil secondary salinization formation and provide ideas for prevention of secondary salinization under long-term continuous cropping.

Effect of aluminum stress on the absorption and transportation of aluminum and macronutrients in roots and leaves of Aleurites montana

Aluminum toxicity is common in subtropical acidic soils, and the effect of Al stress on Aleurites montana is unknown. This study determined the growth and Al and macronutrient contents (C, N, P, K, Ca, and Mg) in leaves and roots of 1-year-old A. montana seedings to reveal the effects of Al stress on growth and elements of A. montana seedings and the difference of growth and content of elements in potted seedlings and hydroponic seedlings under Al addition. The content of Al in the leaves of potted A. montana seedlings was greater than 1000 mg kg−1 under Al stress, so it can be judged that A. montana is likely an Al accumulator and has a high Al tolerance in potted soil. The changes of macronutrient contents in the seedlings varied with Al concentration. For potted seedlings, Al stress had a significant effect on Mg and K contents in roots and Mg and C contents in leaves, but it had no effect on other macronutrients, such as N, P and Ca. For hydroponic seedlings, Al content in roots and N, P, and K contents in roots and leaves increased significantly, and C and Ca contents in roots decreased significantly when the Al concentration exceeded 0.290 mmol L−1. The Al stress affected the distribution and transportation of macronutrients between roots and leaves. The Al and K contents in the roots were higher than those in leaves in both cultivation methods, while the C, N, and Ca contents were smaller than those in leaves. The transportation of N, P and Ca from root to leaf in potted seedlings changed with the increase of Al concentration. There were major differences in the growth and the absorption and transportation of elements between potted and hydroponic seedlings under Al stress. In this study, Al promoted or had no significant effect on the growth of potted seedlings, while the growth of hydroponic seedlings was significantly inhibited when the Al concentration was relatively high. Moreover, the effect of Al concentration on Al and macronutrients of potted seedlings was less than that of hydroponic seedlings. Therefore, the effects of the primary environment should be considered in future stress experiments. This study revealed the effect of Al concentration on A. montana seedlings from the perspective of growth and nutrition, which are crucial in ecological restoration function of mixed tree species in acid rain area. Comparison of the effects of Al concentration on A. montana seedlings under two cultivation methods provides new perspectives for future stress test research.

Methane and Nitrous Oxide Flux after Biochar Application in Subtropical Acidic Paddy Soils under Tobacco-Rice Rotation

Biochar amendment is a good means of mitigating methane (CH4) and nitrous oxide (N2O) emissions. However, the effects of biochar amendment on N2O and CH4 reduction in soil under rotation with different soil moisture contents is not well understood. To understand CH4 and N2O flux from soil with biochar amendment under water-unsaturated and water-saturated conditions, a field experiment was conducted in a tobacco-rice rotation field in subtropical China to investigate N2O and CH4 emissions following soil amendment with tobacco straw biochar at rates of 0, 10, 40 and 80 t·ha−1 (B0, B10, B40 and B80, respectively). N2O and CH4 emissions were monitored by a closed-chamber method in the water-unsaturated tobacco (UT) and water-saturated rice (SR) seasons during the 2015 planting season. The soil pH increased from 5.4 in the control to 6.1 in the soil amended with biochar at 80 t·ha−1 in the UT season. During both the UT and SR seasons, with biochar amendment at 40 and 80 t·ha−1, the soil bulk density (BD) decreased, while the soil organic matter (SOM) and available potassium (Av. K) contents increased. N2O flux was significantly greater in UT than in SR in the controls but decreased with the application of biochar during both the UT and SR seasons. The cumulative CH4 emission decreased with the rate of biochar application and the methanotroph pmoA gene copy number in soils and increased with the methanogenic archaea 16Sr DNA gene copy number in soils during the rice-cropping season. These results indicated that biochar amendment could decrease methanogenic archaea and increase of methanotroph pmoA gene, which are the mechanistic origin for CH4 reduction.

Manganese accumulation and plant physiology behavior of Camellia oleifera in response to different levels of nitrogen fertilization

Manganese (Mn) pollution in soil, especially around the mining areas, is a severe problem in China. Seeking for effective remediation methods for Mn-contaminated soil is therefore urgent and necessary. Camellia oleifera (C. oleifera) is one of the world's four major woody oil plants, which is widely cultivated in subtropical acidic soils for oil production and has become an important economic and ecological resource in Guangxi Province. Nitrogen (N) is one of the most common limiting factors for plant growth and development in soils. We carried out this study to evaluate the effects of different N fertilization levels (0, 100, 300 and 500 mg kg−1) on the morphological and physiological characteristics of C. oleifera in two soils with different Mn-contamination degrees. The results indicate that N fertilization affected the plant growth and the content of photosynthetic pigments, while C. oleifera accumulated great amounts of Mn in both soils. However, the plant biomass reduced significantly at the high-level N fertilization (≥300 mg kg−1), and the oxidative stress was stimulated under Mn contamination. As a comparison, the plant biomass remained unaffected at the low-level N fertilization (100 mg kg−1), and the ascorbate peroxidase (APX) activity in C. oleifera leaves were enhanced to alleviate the oxidative stress and therefore protecting the plant from Mn contamination. Meanwhile, plants supplemented with a low-level of N fertilizer (100 mg kg−1) had appropriate antioxidant enzyme and nonenzymatic antioxidant activities, which indicates that this was favorable growth conditions for C. oleifera. Thus, the recommended N fertilization level for maintaining plant biomass and increasing Mn accumulation in plant is 100 mg kg−1 N; at which level the efficiency of Mn phytoremediation by C. oleifera can be further enhanced.

Forms and buffering potential of aluminum in tropical and subtropical acid soils cultivated with Pinus taeda L

Several interactions between Al and the solid phase of soil influence Al buffering in soil solution. This work evaluated soils cultivated with Pinus taeda L. to determine Al forms in organic and mineral horizons using various extraction methods and to relate acidity with clay mineralogy. Organic and mineral horizons of 10 soil profiles (up to 2.1 m deep) in southern Brazil were sampled. Organic horizons were separated into fresh, aged, and fermented/humified litter. The following Al extraction methods were utilized: 0.5 mol L−1 pH 2.8 CuCl2–Al complexed in organic matter; 1.0 mol L−1 KCl–exchangeable Al; water–Al soluble in soil solution; HF concentrated + HNO3 concentrated + H2O2 30% (v/v)–total Al. Six sequential extractions were carried out to isolate different forms of amorphous minerals that can buffer Al on soil solution: 0.05 and 0.1 mol L−1 sodium pyrophosphate; 0.1 and 0.2 mol L−1 ammonium oxalate; 0.25 and 0.5 mol L−1 NaOH. Samples of clay were also analyzed by XRD. There was a clear effect of litter age on increasing total Al concentration. In the aged litter and fermented and/or humified litter, levels of total Al were 1.4 to 3.8 and 1.5 to 7.8 times greater than in fresh litter, respectively. The CuCl2 method had higher Al extraction capacity than the KCl method for litter. The lowest Al–pyrophosphate values were observed in the Oxisol, which also had a predominance of gibbsite and the lowest levels of Al–KCl and Al–CuCl2. There was an inverse relationship between degree of soil weathering and soluble and exchangeable Al in soils. Available Al increased with higher Si proportion in minerals of the clay fraction (2:1 > 1:1 > 0:1). The worst scenario was soils with the combination of high soluble and exchangeable Al levels and high concentrations of amorphous forms of Al minerals. The best predictors of Al accumulation in the youngest litter horizon were extractions of amorphous minerals with pyrophosphate and NaOH. These extractors are normally used to predict the level of Al buffering in soils. Organic matter had less influence on Al dynamics in soils.

Land-use type affects nitrate production and consumption pathways in subtropical acidic soils

The production and consumption pathways of nitrate (NO3−-N) are key factors for NO3−-N retention capacity (NR) in soils, especially in subtropical regions with high precipitation. This study was designed to examine the effect of land-use type on NO3−-N production and consumption pathways, in acidic soils in a subtropical region. Soils were collected from 5 differing land-use types: shrubland (SB), a eucalyptus plantation (ET), a sweet potato farm (SP), a citrus orchard (CO) and a mixed-vegetable farm (VE), in southeast China. In our study, SB was considered a natural system, ET was considered an artificial plantation system, and SP, CO and VE were considered agricultural systems. An incubation experiment was conducted with two 15N tracing treatments, and a numerical modeling method was applied to the experimental data to quantify gross N transformation rates. The results showed that 76% and 69% of NO3−-N was produced by heterotrophic nitrification (ONrec) in SB and ET respectively. Whereas, ONrec was negligible in SP, CO and VE, with >99% of NO3−-N produced by autotrophic nitrification. Dissimilatory NO3−-N reduction to NH4+-N (DNRA) had an important role in soil NO3−-N consumption in SB and ET, with >90% of NO3−-N produced in SB being simultaneously consumed via DNRA. However, DNRA was unimportant in SP, CO and VE. Nitrate immobilization and denitrification (INO3) was negligible for all land-use types. Soil NR under different land-use types was in the order SB > ET > SP ≈ CO ≈ VE. Although N fertilizer was applied at low rates, for only the following two years after planting eucalyptus saplings, the NR in ET was significantly lower than in SB (natural system). Nitrate retention capacity was negatively correlated with soil organic carbon (SOC) and total nitrogen (TN) concentrations, and positively correlated soil C:N and C: NO3−-N ratios. The lower NR in SP, CO and VE may therefore be due to higher SOC and TN concentrations and lower C:N and C:NO3−-N ratios in those soils relative to SB and ET soils. Our observations imply that ONrec and DNRA play an important role in soil NO3−-N production and consumption in SB and ET land-uses, but not in agricultural land-use (SP, CO and VE), within the subtropical region of China studied. These findings could contribute to a theoretical and practical framework for managing NO3−-N in subtropical acidic soils.

Effects of reforestation on ammonia-oxidizing microbial community composition and abundance in subtropical acidic forest soils.

Forest ecosystems have great ecological values in mitigation of climate change and protection of biodiversity of flora and fauna; re-forestry is commonly used to enhance the sequestration of atmospheric CO2 into forest storage biomass. Therefore, seasonal and spatial dynamics of the major microbial players in nitrification, ammonia-oxidizing archaea (AOA) and bacteria (AOB), in acidic soils of young and matured revegetated forests were investigated to elucidate the changes of microbial communities during forest restoration, and compared to delineate the patterns of community shifts under the influences of environmental factors. AOA were more abundant than AOB in both young and matured revegetated forest soils in both summer and winter seasons. In summer, however, the abundance of amoA-AOA decreased remarkably (p < 0.01), ranging from 1.90 (± 0.07) × 108 copies per gram dry soil in matured forest to 5.04 (± 0.43) × 108 copies per gram dry soil in young forest, and amoA-AOB was below detection limits to obtain any meaningful values. Moreover, exchangeable Al3+ and organic matter were found to regulate the physiologically functional nitrifiers, especially AOA abundance in acidic forest soils. AOB community in winter showed stronger correlation with the restoration status of revegetated forests and AOA community dominated by Nitrosotalea devanaterra, in contrast, was more sensitive to the seasonal and spatial variations of environmental factors. These results enrich the current knowledge of nitrification during re-forestry and provide valuable information to developmental status of revegetated forests for management through microbial analysis.

Rapid recovery of nitrogen retention capacity in a subtropical acidic soil following afforestation

Understanding soil nitrogen (N) dynamic and availability during afforestation (the conversion of cropland to forest plantations) is critical to maintain forest growth and long-term productivity, especially in rainfall-rich, subtropical region. However, only few studies have investigated the inherent N transformation processes involved in N availability in subtropical acidic soils. In a 15N tracing study, nine soils from croplands, 10-y and 50-y afforested woodlands were sampled to investigate the changes in soil gross N transformation rates in humid subtropical China. Gross N transformation rates were not significantly different in soils under 10- and 50-y after afforestation. Compared to cropland, however, afforestation stimulated the rates of mineralization, microbial NH4+ immobilization and adsorption of NH4+, leading to a faster turnover of NH4+ pool in afforested soils. Moreover, afforestation inhibited autotrophic nitrification and resulted in NO3− production dominated by heterotrophic nitrification. Furthermore, afforestation significantly enhanced NO3− consumption mainly through the increase in microbial NO3− immobilization rather than dissimilatory NO3− reduction to NH4+ in soil. These differences in gross N transformation rates resulted in low net NO3− production and strong NO3− retention capacity in afforested soils, similar to that found in undisturbed natural forest. Our results suggest a rapid recovery (several years) of soil N retention following afforestation. Soil NO3− retention capacity was correlated positively with TOC, TN, WHC, CEC, Al, free Al oxide and exchangeable Al3+, but negatively with pH, Ca and exchangeable Ca2+, indicating that the absence of agricultural management (e.g., N fertilizer and liming) and alteration in soil environment by tree establishment are responsible for the recovery of N retention capacity in subtropical acidic soils during afforestation.

Land-use type affects N2O production pathways in subtropical acidic soils

The change in land-use from woodland to crop production leads to increased nitrous oxide (N2O) emissions. An understanding of the main N2O sources in soils under a particular land can be a useful tool in developing mitigation strategies. To better understand the effect of land-use on N2O emissions, soils were collected from 5 different land-uses in southeast China: shrub land (SB), eucalyptus plantation (ET), sweet potato farmland (SP), citrus orchard (CO) and vegetable growing farmland (VE). A stable isotope experiment was conducted incubating soils from the different land use types at 60% water holding capacity (WHC), using 15NH4NO3 and NH415NO3 to determine the dominant N2O production pathway for the different land-uses. The average N2O emission rates for VE, CO and SP were 5.30, 4.23 and 3.36 μg N kg−1 dry soil d−1, greater than for SB and ET at 0.98 and 1.10 μg N kg−1 dry soil d−1, respectively. N2O production was dominated by heterotrophic nitrification for SB and ET, accounting for 51 and 50% of N2O emissions, respectively. However, heterotrophic nitrification was negligible (<8%) in SP, CO and VE, where autotrophic nitrification was a primary driver of N2O production, accounting for 44, 45 and 66% for SP, CO and VE, respectively. Denitrification was also an important pathway of N2O production across all land-uses, accounting for 35, 35, 49, 52 and 32% for SB, ET, SP, CO and VE respectively. Average N2O emission rates via autotrophic nitrification, denitrification and heterotrophic nitrification increased significantly with gross nitrification rates, NO3− contents and C:N ratios respectively, indicating that these were important factors in the N2O production pathways for these soils. These results contribute to our understanding and ability to predict N2O emissions from different land-uses in subtropical acidic soils and in developing potential mitigation strategies.

N2O production pathways relate to land use type in acidic soils in subtropical China

Agricultural practises impact soil properties and N transformation rate, and have a greater effect on N2O production pathways in agricultural soils compared with natural woodland soils. However, whether agricultural land use affects N2O production pathways in acidic soils in subtropical regions remains unknown. In this study, we collected natural woodland soil (WD) and three types of agricultural soils, namely upland agricultural (UA), tea plantation (TP) and bamboo plantation (BP) soils. We performed paired 15N-tracing experiment to investigate the effects of land use types on N2O production pathways in acidic soils in subtropical regions in China. The results revealed that heterotrophic nitrification is the dominant pathway of N2O production in WD, accounting for 44.6 % of N2O emissions, whereas heterotrophic nitrification contributed less than 2.7 % in all three agricultural soils, due to a lower organic C content and soil C/N ratio. In contrast, denitrification dominated N2O production in agricultural soils, accounting for 54.5, 72.8 and 77.1 % in UA, TP and BP, respectively. Nitrate (NO3 −) predominantly affected the contribution from denitrification in soils under different land use types. Autotrophic nitrification increased after the conversion of woodland to agricultural lands, peaking at 42.8 % in UA compared with only 21.5 % in WD, and was positively correlated with soil pH. Our data suggest that pH plays a great role in controlling N2O emissions through autotrophic nitrification following conversion of woodland to agricultural lands. Our results demonstrate the variability in N2O production pathways in soils of different land use types. Soil pH, the quantity and quality of organic C and NO3 − content primarily determined N2O emissions. These results will likely assist modelling and mitigation of N2O emissions from different land use types in subtropical acidic soils in China and elsewhere.

Are nitrate production and retention processes in subtropical acidic forest soils responsive to ammonium deposition?

Changes in soil N-cycling and retention processes in subtropical/tropical acidic forest ecosystems under anthropogenic N inputs are not well understood. We conducted a laboratory 15N tracing study on an acid soil (pH values: 4.6 to 5.0) from a subtropical forest fertilized for more than 2.5 years at a rate of 0, 40, and 120 kg NH4ClN ha−1 yr−1, respectively. To get a better resolution of mechanistic changes in soil N cycling and retention processes under NH4+ additions, we used a conceptual 15N tracing model to quantify process-specific and pool-specific N transformation rates in soils. Gross N mineralization rates decreased at high NH4+ additions, which were paralleled by a reduction in fungal biomass and mineralization of recalcitrant organic N. Gross NH4+ immobilization rates did not show a change with increasing NH4+ additions. Interestingly, soil NO3− production (heterotrophic, autotrophic, and gross nitrification) and retention (NO3− immobilization and dissimilatory nitrate reduction to ammonium) showed insensitivity to increasing additions of NH4+. The mechanisms behind the lack of response of heterotrophic nitrification were unclear, but possibly related to the absence of significant changes in soil C: N ratio and soil acidity under increased NH4+ additions. Because of the low autotrophic nitrification potential and the lack of NH4+ limitation to autotrophic nitrifiers, autotrophic nitrification was unresponsive to NH4+ additions. NO3− immobilization rates appeared to be controlled by the NO3− produced from heterotrophic nitrification, as indicated by the positive relationship between NO3− immobilization and heterotrophic nitrification (R2 = 0.59, p = 0.015), thus showing a lack of a change under increased NH4+ additions. DNRA seemed to be inherently less responsive to environmental changes such as NH4+ deposition. Our work demonstrates that enhanced NH4+ deposition has a low potential to stimulate soil NO3− production and weaken soil retention of NO3− in this, and perhaps other subtropical/tropical acidic forest ecosystems.

Co-occurrence of nitrite-dependent anaerobic ammonium and methane oxidation processes in subtropical acidic forest soils

Anaerobic ammonium oxidation (anammox) and nitrite-dependent anaerobic methane oxidation (n-damo) are two new processes of recent discoveries linking the microbial nitrogen and carbon cycles. In this study, 16S ribosomal RNA (rRNA) gene of anammox bacteria and pmoA gene of n-damo bacteria were used to investigate their distribution and diversity in natural acidic and re-vegetated forest soils. The 16S rRNA gene sequences retrieved featured at least three species in two genera known anammox bacteria, namely Candidatus Brocadia anammoxidans, Candidatus Brocadia fulgida, and Candidatus Kuenenia stuttgartiensis while the pmoA gene amplified was affiliated with two species of known n-damo bacteria Candidatus Methylomirabilis oxyfera and a newly established Candidatus Methylomirabilis sp. According to the results, the diversity of anammox bacteria in natural forests was lower than in re-vegetated forests, but no significant difference was observed in n-damo community between them. Quantitative real-time PCR showed that both anammox and n-damo bacteria were more abundant in the lower layer (10-20cm) than the surface layer (0-5cm). The abundance of anammox bacteria varied from 2.21×10(5) to 3.90×10(6) gene copies per gram dry soil, and n-damo bacteria quantities were between 1.69×10(5) and 5.07×10(6) gene copies per gram dry soil in the two different layers. Both anammox and n-damo bacteria are reported for the first time to co-occur in acidic forest soil in this study, providing a more comprehensive information on more defined microbial processes contributing to C and N cycles in the ecosystems.

Effects of leaf litter tannin on soil ammonium and nitrate content in two different forest soils of mount Wuyi, China

ABSTRACTIn forest ecosystems, plant secondary metabolites, such as tannin, affect soil net nitrogen (N) mineralisation and nitrification. However, the mechanisms underlying the effects of tannin in plant leaf litter on N transformation in subtropical acidic soils containing high levels of inorganic N in the form of NH4+-N occurs. An incubation experiment was carried out using two litter types and tannin acid addition into Hapludults (Hu) and Haplohumults (Ho) soils in order to determine if (i) addition of leaf litter with higher tannin content might decrease soil N content, and (ii) effect modes of litter or tannin addition on NH4+-N and NO3−-N are different. The net production rate, relative change rate and relationship between effects of tannin and litters on NH4+-N or NO3−-N were analyzed. Our results suggested that addition of each species litter into forest soil decreased soil inorganic N content, and the effects on NO3−-N and NH4+-N dynamics were related to tannin content of the litter. Further, soil inorganic N content was reduced at a higher rate in the short term for NH4+-N than for NO3−-N. Inorganic N loss was dependent on the form of carbon released at different stages of litter decomposition. Microbial immobilisation of N did not completely account for the marked fall in N. In forest soils, the influence of plant litter tannin content on N transformations including nitrification and immobilisation were related to soil types. Our study enhances our understanding of how plant litter quality influences N transformation in forest ecosystems.

Manganese oxide affects nitrification and ammonia oxidizers in subtropical and temperate acid forest soils

Variations in nitrification dynamics were observed both in temperate and subtropical acidic forest soils. The effect of Mn on nitrification was studied to explain variation in nitrification between different soil types. Weakly and highly acidic soils in subtropical and temperate forests were treated with 0% or 3% birnessite. The nitrification process was simulated by kinetic model. Dynamic changes of amoA gene abundance of ammonia monooxygenase (AMO) for ammonia-oxidizing bacteria (AOB) and ammonia-oxidizing archaea (AOA) were estimated by qPCR. Nitrification rates were significantly decreased by Mn addition in 3days both for the weakly acidic subtropical and temperate soils. However, the total nitrification rate did not change for either soil by Mn addition after 10days' incubation. Nitrification was best fitted by a first-order kinetic model for both weakly acidic soils. However, it was best fitted with a zero-order model after MnO2 addition. Numbers of AMO amoA gene copy decreased after Mn addition. No significant nitrification was observed for highly acidic soils and Mn had a minimal effect. Soil nitrification was retarded by 3% MnO2 for both subtropical soil and temperate soils. Nitrification dynamics were altered by MnO2 in weakly acidic soils, probably due to Mn toxicity.

Effects of land use change on soil gross nitrogen transformation rates in subtropical acid soils of Southwest China.

Land use change affects soil gross nitrogen (N) transformations, but such information is particularly lacking under subtropical conditions. A study was carried out to investigate the potential gross N transformation rates in forest and agricultural (converted from the forest) soils in subtropical China. The simultaneously occurring gross N transformations in soil were quantified by a (15)N tracing study under aerobic conditions. The results showed that change of land use types substantially altered most gross N transformation rates. The gross ammonification and nitrification rates were significantly higher in the agricultural soils than in the forest soils, while the reverse was true for the gross N immobilization rates. The higher total carbon (C) concentrations and C / N ratio in the forest soils relative to the agricultural soils were related to the greater gross N immobilization rates in the forest soils. The lower gross ammonification combined with negligible gross nitrification rates, but much higher gross N immobilization rates in the forest soils than in the agricultural soils suggest that this may be a mechanism to effectively conserve available mineral N in the forest soils through increasing microbial biomass N, the relatively labile organic N. The greater gross nitrification rates and lower gross N immobilization rates in the agricultural soils suggest that conversion of forests to agricultural soils may exert more negative effects on the environment by N loss through NO3 (-) leaching or denitrification (when conditions for denitrification exist).